1. Field of the Invention
The present invention relates, in general, to hearing devices and, in particular, to implantable hearing devices.
2. Description of the Related Art
With the advent of the operating microscope in the 1950's a newly born field of otology and microsurgery emerged. The development of tympanoplasty and stapedectomy have led to the amelioration of many varieties of conductive hearing loss. Even some sensorineural hearing losses have been successfully treated using microsurgical techniques.
Despite these surgical accomplishments, however, 20 million people in the United States of America still suffer from various degrees of hearing loss. While the vast majority of the people in this group are candidates for conventional hearing aid use, in reality only approximately 15 percent becomes users of an amplification device to overcome their hearing loss. Several factors, such as sound distortion, discomfort of fit, and cosmetic appearance are ostensibly to blame for this low incidence of conventional hearing aid use.
Volta, in 1800, first introduced the concept of stimulating the ear by using electricity. The carbon hearing aid first appeared about the turn of the century. Another landmark development was the vacuum tube hearing aid, introduced in the late 1930's, which, in time, was supplanted in the 1950's by the transistor-operated systems. All conventional hearing aids now use transistors and integrated circuits to improve frequency response, reduce size, lower harmonic distortion and increase flexibility of fit as compared to their predecessors, yet perhaps not to the ultimate satisfaction of each potential user.
The conventional hearing aid is composed of a microphone, an amplifier, a battery as a power source, and a speaker or earphone (commonly referred to as a receiver in the hearing aid industry). The implantable hearing device has the same basic components, except that the speaker is replaced by a driving vibrating component, such as an electromagnetic coil or a piezoelectric system of bimorph design. Environmental sound energy, as it passes through either device, is converted by the microphone into an electrical signal which is routed to an amplifier. In the conventional hearing aid, the speaker transduces the amplified electrical signals into acoustic energy, which is then transmitted to the tympanic membrane and ossicular chain. In the implantable hearing device, the speaker is eliminated, being replaced by the vibratory component which drives the ossicular chain.
Some investigators have directed their efforts to the needs of the hard-of-hearing patient, designing systems intended to circumvent many of the problems of the conventional hearing aid. Rutschmann, et al, in 1959, described auditory stimulation by the use of alternating magnetic fields acting on permanent magnets fixed to the ear drum. In 1967, investigators at the University of Pittsburgh, Department of Electrical Engineering, designed the first implantable hearing aid of this type. A U.S. Patent was granted but no further animal or human research has been reported.
U.S. Pat. No. 3,209,081 to Ducote, et al. discloses a hearing aid in which a sound amplifier and transmitter unit, carried on the body of the user, is used to transmit radio frequency signals to a remote receiver that is implanted subcuntaneously against the skull of the user in a concealed position.
U.S. Pat. No. 3,346,704 to Mahoney discloses a hearing aid unit adapted to be implanted within the mastoid antrum of the user. The unit comprises a microphone, a battery, an amplifying system, and a speaker. A microphone tube extends from the microphone to a point beneath the skin and behind the ear of the user to transmit sound from the environment to the microphone, while a speaker tube extends from the speaker through the mastoid antrum into the middle ear space behind the ear drum to transmit the sound thereto.
Wingrove, U.S. Pat. No. 3,594,514, discloses an implantable hearing aid having a piezoelectric ceramic element mounted adjacent to the auditory conductive system of the middle ear to impart vibration thereto. Electrical circuitry to the piezoelectric element provides electrical signals representative of sound waves.
Goode's 1970 article on the state of the art in implantable hearing aids rekindled interest in this approach and progress continued throughout the early 1970's through the work of Glorig, et al., and Fredrickson, et al. who explored the feasibility of different implantable systems. Patent activity in that time frame includes U.S. Pat. No. 3,764,748 to Branch, et al; U.S. Pat. No. 3,870,832 to Fredrickson, and U.S. Pat. No. 3,882,285 to Nunley, et al.
Branch, et al., supra, discloses several hearing aid configurations for implantation within the middle ear cavity interiorly of the ear drum to the ossicle bone chain. Auditory signals picked off the ear drum are subsequently amplified and/or transmitted to natural and/or solid-state sound receiving mechanisms located on the oval window, the round window, or the promontory leading into the inner ear. The tensor tympani and stapedial muscles prevent loud sounds from damaging the inner ear.
Fredrickson, supra, employs an implanted coil and magnet in the ear after removal of the incus. The magnet is fastened to the head of the stapes and the coil, when energized by electrical signals from a sound transducer, produces a magnetic field which interacts with the magnetic field of the magnet. This interaction of the two magnetic fields causes movement of the stapes in the same manner as it is normally activated by the incus.
Nunley, et al., supra, discloses an implantable hearing aid which is implanted in a hollowed-out portion of the skull adjacent to the ear canal. A microphone part is connected to the ear canal which receives sound that enters the ear and transforms it into energies which are transmitted via mechanical means to the movable portions of the middle ear. In this fashion, a parallel sound path is provided which augments and supports the transmission of sound to improve hearing.
Advances in technology in the 1980's have spurred additional efforts. Suzuki, et al. and Yanagihara, et al. have published reports which describe middle ear implants in animals and humans using a piezoelectric vibrator of bimorph design. Several of their patients have reported good amplification and high fidelity of sound perception; indeed, the efforts of the Japanese group constitutes a major breakthrough in implantation devices.
Tjellstrom, et al., in 1981, 1983 and 1985, developed another variation of an implantable device. This system used a bone condition "hearing aid" anchored directly to the temporal bone by implanted osseo-integrated titanium screws which exit to the surface percutaneously. While the incidence of infection has been low in the implanted subjects, the device has not been received altogether enthusiastically by many patients, ostensibly due to its design. It is helpful in patients with 45 dB or better bone conduction. In 1985, Hough, et al. introduced a modification of the temporal bone stimulator for patients with bone conduction thresholds of 25 dB hearing loss or better. In this device, the titanium bone conduction vibrator is screwed into the temporal bone and no electronics are used except for a radio link coils to transmit the electrical signal transcutaneously. A vibrating coil activates the screw implanted in the bone. An ear-level and body-borne variant of this system have been presented to date. Although the body-borne system provides about 10 dB more gain than the ear level device, it has been less accepted by patients than the ear level device. See U.S. Pat. No. 4,606,329 to Hough for further details. Neither device (body-borne or ear level) has been quite as efficient as the Tjellstrom implant, perhaps because they employ indirect coupling through radio frequency transmission rather than direct bone conduction stimulation. Revisions of the electronic design may very well improve the efficiency of the bone conduction temporal bone stimulator of Hough, and since the device has U.S. Food and Drug Administration (FDA) approval, it should receive considerable clinical application in the United States and abroad. For a description of a particular type of coupling to a bone-anchored hearing aid, see U.S. Pat. No. 4,498,461 to Hakansson.
In 1987, Hough et al. reported on a middle ear implantable hearing device using electromagnetic principles applied to humans undergoing middle ear surgery under local anesthesia. Although the device was functional, its electrical power consumption was excessive.
Ko, Maniglia and Zhang also reported, in 1987, their experience with an electromagnetic middle ear hearing aid using direct stimulation of the stapes. Goode, et al. have experimented with a piezoelectric system to produce stapes vibration in fresh human temporal bones. Heide, et al. in 1988 presented the advantages of an electromagnetic hearing aid in the ear canal driving a magnet glued to the ear drum. Finally, Goode has recently reported encouraging results with another design in which another electromagnetic canal device similar in principle and design that stimulates a samarium cobalt magnet glued to the ear drum. However, the magnet glued to the ear drum only stays in place temporarily, and a better system is necessary.
Thus, while technological advances in the conventional hearing aid industry have succeeded in miniaturizing the components and improving the efficiency and gain of these devices, there are still problems to be solved. Some of the drawbacks associated with the conventional hearing aid are: high internal noise, acoustic feedback, limited fidelity due to sound distortion and a limited frequency response range. Additionally, many hearing aid styles are considered to be cosmetically unattractive, overly conspicuous, or even uncomfortable if they employ a tightly fitting earmold. As a consequence, a highly efficient, totally concealed conventional hearing aid is not yet available. Finally, the candidacy of a given person for hearing aid use may also be restricted on medical grounds because of such problems as chronic middle ear infections, stenosis or atresia of the external auditory canal, or prior radical mastoidectomy.
In theory, the attractiveness of an implantable hearing device lies in its ability to overcome many of the drawbacks mentioned above for the conventional hearing aid. However, if any implantable device is to provide a viable alternative to the conventional hearing aid, the device must not only overcome many of these drawbacks but it must also minimize all potential risk factors introduced by the surgical procedure. It should be made out of biocompatible materials and have the lowest risk possible regarding middle ear or inner ear complications. Ideally, its technical design should be trouble-free for many years so that the need for revision surgery is minimal. Finally, it should have the following advantages: (1) totally concealed cosmetically; (2) highly efficient power consumption; (3) high sound fidelity; (4) broad, flat frequency response; (5) minimal sound distortion; (6) elimination of ringing feedback; (7) adequate acoustic gain; and (8) be flexible and versatile so as to be applicable to cases involving conductive as well as sensorineural hearing loss, and to all age groups, especially the pediatric age group that would probably derive the greatest benefit from such a device, because of many more years of potential use.